Abstract

This study analyses the evaluation of tomographic indicators of tibia structure, assuming that the usual loading pattern
shifts from uniaxial compression close to the heel to a combined compression, torsion and bending scheme
towards the knee. To this end, pQCT scans were obtained at 5% intervals of the tibia length (S5–S95 sites from heel
to knee) in healthy men and women (10 ⁄ 10) aged 20–40 years. Indicators of bone mass [cortical area, cortical ⁄ total
bone mineral content (BMC)], diaphyseal design (peri ⁄ endosteal perimeters, cortical thickness, circularity, bending
⁄ torsion moments of inertia – CSMIs), and material quality [(cortical vBMD (bone mineral density)] were determined.
The longitudinal patterns of variation of these measures were similar between genders, but male values were
always higher except for cortical vBMD. Expression of BMC data as percentages of the minimal values obtained along
the bone eliminated those differences. The correlative variations in cortical area, BMC and thickness, periosteal
perimeter and CSMIs along the bone showed that cortical bone mass was predominantly associated with cortical
thickness toward the mid-diaphysis, and with bone diameter and CSMIs moving more proximally. Positive relationships
between CSMIs (y) and total BMC (x) showed men’s values shifting to the upper-right region of the graph and
women’s values shifting to the lower-left region. Total BMC decayed about 33% from S5 to S15 (where minimum
total BMC and CSMI values and variances and maximum circularity were observed) and increased until S45, reaching
the original S5 value at S40. The observed gender-related differences reflected the natural allometric relationships.
However, the data also suggested that men distribute their available cortical mass more efficiently than women. The
minimum amount and variance of mass indicators and CSMIs, and the largest circularity observed at S15 reflected the
assumed adaptation to compression pattern at that level. The increase in CSMIs (successively for torsion, A–P bending,
and lateral bending), the decrease in circularity values and the changes in cortical thickness and periosteal perimeter
toward the knee described the progressive adaptation to increasing torsion and bending stresses. In agreement
with the biomechanical background, the described relationships: (i) identify the sites at which some changes in tibial stresses and diaphyseal structure take place, possibly associated with fracture incidence; (ii) allow prediction of mass
indicators at any site from single determinations; (iii) establish the proportionality between the total bone mass at
regions with highly predominant trabecular and cortical bone of the same individual, suitable for a specific evaluation
of changes in trabecular mass; and (iv) evaluate the ability of bone tissue to self-distribute the available cortical
bone according to specific stress patterns, avoiding many anthropometric and gender-derived influences.